The present invention is directed to methods for removing lower alkyl alcohol from polyol fatty acid polyesters by contacting the polyol fatty acid polyester and lower alkyl alcohol mixture with a stripping mixture comprising an inert stripping gas, lower alkyl alcohol and oxygen. After such contact, the concentration of lower alkyl alcohol in the stripping mixture is increased. The invention is also directed to energy efficient methods for removing the lower alkyl alcohol from the stripping mixture by increasing the pressure and reducing the temperature of the stripping mixture so that a portion of the lower alkyl alcohol condenses to a liquid which can be more easily separated from the stripping mixture.
There has been considerable interest in the use of certain polyol fatty acid polyesters as low or reduced calorie substitutes for fats and oils in foods. For example, non-absorbable, non-digestible sugar fatty acid esters or sugar alcohol fatty acid esters having at least four fatty acid ester groups, with each fatty acid having from 8 to 22 carbon atoms, have been used as partial or full fat substitutes in low calorie food compositions.
A number of different processes have been disclosed in the art for preparing these highly esterified polyol fatty acid polyesters, in particular sucrose polyesters. In general, a polyol, for example sucrose, is reacted with a fatty acid lower alkyl ester in the presence of a basic initiator catalyst to form a polyol fatty acid polyester. Emulsifiers, phase transfer catalysts and the like can be used to promote the reaction between the polyol and the fatty acid lower alkyl ester. Lower alkyl alcohol is a by-product of this reaction and its presence in the reaction mixture tends to slow the reaction""s progression. Additionally, the lower alkyl alcohol is generally not a desired component in the polyol fatty acid polyester product. Thus, it is desirable to remove the lower alkyl alcohol from the polyol fatty acid polyester to both produce a purified polyol fatty acid polyester and to speed the reaction between a polyol and a fatty acid lower alkyl ester.
Moreover, oxygen is generally considered a poison in a transesterification reaction or in a mixture of a polyol fatty acid polyester and lower alkyl alcohol due to its tendency to oxidize reactants and products and to degrade the reaction catalyst. Hence, it is desirable to minimize the amount of oxygen which is added to a mixture of polyol fatty acid polyester and lower alkyl alcohol.
Lower alkyl alcohol can be removed from a polyol fatty acid polyester by sparging with an inert gas. This process is discussed in U.S. Pat. No. 4,518,772 to Volpenhein, U.S. Pat. No. 3,963,699 to Rizzi et al. and U.S. Pat. No. 4,517,360 to Volpenhein. These patents disclose a method of vacuum separation for the removal of the lower alkyl alcohol wherein inert gas sparging is used as a supplement to the vacuum removal of lower alkyl alcohol.
Processes for removing volatile organics from inert gas streams are generally known. For example, U.S. Pat. No. 4,295,282 to Fox discloses an open cycle heat pump system for recovering condensable solvents and/or heat from gas streams. This process is disclosed in conjunction with the removal of paint fumes and other volatile vapors from the gas stream. U.S. Pat. No. 4,480,393 to Flink et al. and U.S. Pat. No. 5,152,812 to Kovach also disclose processes for recovering condensable organic components from an inert gas stream.
Owing to the increased use of polyol fatty acid polyesters in food products and the like, there is a continuing need to improve the efficiency of and reduce the costs of manufacturing the polyol fatty acid polyesters. Specifically, there is a continuing need for fully integrated processes which are both efficient and economical and which can provide for removal of lower alkyl alcohol from a polyol fatty acid polyester mixture comprising a polyol fatty acid polyester and a lower alkyl alcohol.
It is an object of the present invention to provide improved methods for removing lower alkyl alcohol from a polyol fatty acid polyester mixture using a stripping mixture comprising an inert stripping gas, lower alkyl alcohol and oxygen and subsequently removing a portion of the lower alkyl alcohol from the stripping mixture, whereby the stripping mixture may be recycled for further use if desired.
It is an additional object of the present invention to provide such processes for removing a sufficient amount of the lower alkyl alcohol from the stripping mixture to allow the gas to be recycled for further contact with a polyol fatty acid polyester reaction mixture and/or polyester mixture containing lower alkyl alcohol.
In one embodiment, the present invention is directed to a method for removing lower alkyl alcohol from a polyester mixture which comprises polyol fatty acid polyester and lower alkyl alcohol. The polyester mixture is contacted by a stripping mixture comprising an inert stripping gas, up to about 10,000 ppm lower alkyl alcohol and up to about 2000 ppm oxygen. During the contact, at least a portion of the lower alkyl alcohol is transferred from the polyester mixture to the stripping mixture, thereby increasing the concentration of lower alkyl alcohol in the stripping mixture. The stripping mixture is then separated from the polyester mixture and the stripping mixture is compressed to increase its pressure. Additionally, the stripping mixture is cooled to condense at least a first portion of the lower alkyl alcohol to a liquid. Finally, condensed lower alkyl alcohol is separated from the stripping mixture to reduce the amount of lower alkyl alcohol in the stripping mixture to a range of from about 1 ppm to about 10,000 ppm, and the resulting stripping mixture is directed to an expansion turbine in which the temperature and pressure of the stripping mixture are reduced. The energy resulting from the decrease in the stripping mixture temperature and pressure in the expansion turbine is used to compress the stripping mixture separated from the polyester mixture. The resulting stripping mixture may optionally be used as a coolant in one or more heat exchangers to cool an additional quantity of stripping mixture separated from the reaction mixture.
In a preferred embodiment, the present invention comprises a method for removing a lower alkyl alcohol from a polyester mixture in a reaction mixture resulting from the reaction of polyol and fatty acid lower alkyl ester. The reaction mixture comprises reaction products including polyol fatty acid polyester and lower alkyl alcohol. The reaction mixture is contacted by a stripping mixture comprising an inert stripping gas, up to about 10,000 ppm lower alkyl alcohol and up to about 2,000 ppm oxygen. During the contact, at least a portion of the lower alkyl alcohol is transferred from the reaction mixture to the stripping mixture, thereby increasing the concentration of lower alkyl alcohol in the stripping mixture. The stripping mixture is then separated from the reaction mixture and the stripping mixture is compressed to increase its pressure. Additionally, the stripping mixture is cooled to condense at least a first portion of the lower alkyl alcohol to a liquid. Finally, a portion of the condensed first portion of lower alkyl alcohol is separated from the stripping mixture to reduce the amount of lower alkyl alcohol in the stripping mixture to a range of from about 1 ppm to about 10,000 ppm, and the resulting stripping mixture is directed to an expansion turbine in which the temperature and pressure of the stripping mixture are reduced. The energy resulting from the decrease in the stripping mixture temperature and pressure in the expansion turbine is used to compress the stripping mixture separated from the polyester mixture.
In another embodiment, the present invention comprises a method for synthesizing polyol fatty acid polyester. Specifically, a polyol is reacted with fatty acid lower alkyl ester to form a reaction mixture which comprises polyol fatty acid polyester and lower alkyl alcohol. The reaction mixture is contacted with a stripping mixture according to the methods described above.
The methods described herein provide the advantage of an integrated process for removing lower alkyl alcohol from a polyol fatty acid polyester mixture, condensing the alcohol to a liquid and separating the condensed alcohol from the stripping mixture, with significant energy recovery. Surprisingly, it has been found that to remove lower alkyl alcohol from a polyester mixture, an inert stripping gas which comprises up to about 10,000 ppm of lower alkyl alcohol can be used to obtain substantial removal of the lower alkyl alcohol from the polyester mixture. Moreover, in many polyester mixtures oxygen is considered a contaminant and it has been determined that the stripping mixtures of the present invention can comprise up to about 2,000 ppm oxygen without significantly degrading the reaction performance.
Additionally, the stripping mixture may subsequently be vented to the atmosphere or recycled for contact with further polyol fatty acid polyester. By using the cooled stripping mixture from which a portion of the lower alkyl alcohol has been condensed and separated as a coolant in at least one heat exchanger to cool an additional quantity of stripping mixture, additional energy and operational savings may be realized.